Baggage or freight carried on commercial aircraft is often loaded into standardized pallets and containers that are sized to fit efficiently within either a lower lobe or main cargo bay of the aircraft. The pallets or containers are separately filled with the cargo, transported to the aircraft, and loaded onto a handling system that is built into the cargo bay floor. In the conventional cargo handling system, rotatably driven pneumatic tires are mounted under the floor, protruding above the floor a predefined distance to control traction applied to the bottom of the pallets or containers to move them into position within the cargo bay.
The pneumatic tires presently used in cargo handling systems are subject to a number of problems that tend to degrade the operating efficacy of the system. For example, as an aircraft climbs to high altitude, the drop in ambient air pressure increases air loss from the tires due to leakage, causing the inflation pressure of the tires to drop below a recommended level when the aircraft returns to ground. The loss of air pressure in the pneumatic tire reduces the maximum traction or thrust that the tire can apply to move a container, because as the tire inflation pressure drops, the weight of the load is transferred from the tires onto adjacent hard rollers. The tops of the hard rollers define a roller plane that is a few tenths of an inch below the tops of the unloaded pneumatic tires; an underinflated pneumatic tire can allow a heavy container to settle onto the hard rollers since the weight of the load too easily compresses the tire. In addition, underinflated pneumatic tires are more susceptible to damage from impacts with the edges of pallets and containers. Such damage is likely to rupture the tire or puncture it, causing it to go flat. Proper maintenance of the cargo handling system requires frequent checks of the tire inflation, and all too often may involve replacement of flat tires in the system.
Maintenance personnel sometimes attempt to compensate for the expected air loss in pneumatic tires by initially overinflating them. While the higher-than-recommended inflation pressure may provide a short-term solution to this problem for relatively heavier loads, lightweight containers do not cause sufficient vertical deflection of the overinflated tire. Consequently, lightweight containers may ride so high above the roller plane that a flange provided around the base of each container fails to engage hold down clamps that are used to lock the containers in place.
The requirement for servicing and replacing pneumatic tires in cargo handling systems is recognized as a significant cause of aircraft out-of-service time. Commercial carriers have long complained that pneumatic tires are too susceptible to variations in thrust and vertical deflection as a result of changes in inflation pressure, and are not rugged enough to withstand the abrasion and impact forces developed in the aircraft cargo handling system. While it may appear that a wheel assembly using a solid tire would be a likely replacement to overcome these problems, conventional solid tire wheel assemblies have been found incapable of controlling vertical deflection within an acceptable range for the different loads encountered in a cargo handling system--such tires have either deflected too much under heavy loads or not enough under light loads. Furthermore, in conventional wheel assemblies of this type, it was found virtually impossible to adhesively attach a solid tire to a rim so that it would not eventually come free of the rim and slip when applying thrust to a container. The present invention was developed to address these problems.